If you're asking why software has more frequent rebuild cycles than, say, buildings, or roads, or plumbing, it's because it's way cheaper and easier, can be distributed at scale for ~free (compared to road designs which necessarily have to be backward compatible since you can't very well replace every intersection in a city simultaneously), and for all the computerification of the modern world, is largely less essential and less painful to get wrong than your average bridge or bus.
It's like the difference between building a Bird house, Dog house, people house, mansion, large building and a sky scraper... it's different levels of planning, preparation and logistics involved. A lot of software can (or at least should) be done at the bird or dog house level... pretty easy to replace.
